The answer to this question can be found in a very close reading of the text, and also in using inference, or an educated guess, to figure out where he is. At the beginning of the story, he starts out by addressing the audience and telling them why he wants to tell his tale:

So, wherever he is, he knows that tomorrow he is going to die, and he wants to let everyone know what happened before he does. This implies that he is possibly in jail, about to be executed the next day. Either that, or he is going to kill himself, and wants to share his story before he does that. So, given those two alternatives, let's look at what happens to him at the end of the story that he tells. He is with the cops, in his house, so confident of getting away with his crime that he hammers on the walls to show them how sound they are. Unfortunately for him, he hammers through the wall and the rotting corpse of his wife falls out. From this we can assume that he was arrested--they had the body right in front of them, so it was pretty solid proof.

Piecing together what happened at the end, and the fact that he says he is going to die the next day and so much tell his story, I would guess that he is in prison, about to be executed for the crime of murdering his wife. His story is his last confession before he dies. I hope that those thoughts helped; good luck!